From U.S. Pat. No. 5,404,673, a window lifter with a drive to raise and lower a window pane and with an entrapment protection device is known. The speed of the drive and thus the opening and closing speed of the window pane, as well as the direction of movement and position of the window pane, are detected. Upon entrapment of a body part or object between the top edge of the window pane and the door frame, the load on the drive increases, and the drive speed drops below a predefined value. The drive turns off and possibly reverses, and results in the stopping or opening of the window pane.
Since upon entry of the window pane into the door seal before complete closing of the window pane, the drive speed drops to the point of stopping the drive because of the increased resistance, the pane position must be accurately detected, so that the entrapment protection turns off in the seal zone.
For this, a sensor for sensing a position and direction of rotation is provided. The sensor for sensing the direction of rotation consists of a magnetic disk with a north and south pole as well as two Hall sensors offset an angle of 90.degree. relative to each other around the axis of the magnetic disk connected to the drive shaft, which emit sensor signals offset from each other by one-fourth period, from which the direction rotation and thus the direction of movement of the window pane is determined.
The position sensor consists of an annular multipole magnet connected to the drive shaft with alternatingly magnetized magnetic poles and two Hall sensors, which are disposed at a distance of one-half magnetic pole from each other. The magnetization alternation detected by the Hall sensors during a rotation of the drive, and with it that of the annular multipole magnet, are fed as counting pulses to a counter along with the sensor signal of the direction of rotation sensor, whereby the counting pulses are counted upward or downward depending on the direction rotation of the drive, and thus indicate the respective position of the window pane.
For detection of the speed, direction of movement, and position of the window pane, the known drive control and entrapment protection device requires two magnetic disks as signal generators with four Hall sensors. The signal generator provided to trigger the entrapment protection criterion by reducing the speed of the drive has only low resolution with one pole change per revolution.
For speed control of rotating drives, or with a linear adjustment such as a seat distance adjustment to obtain a constant adjustment speed over the adjustment path, a high-resolution sensor system is necessary to enable short reaction times in the control process. However, partitioned signal generators, such as multipole magnets, are subject to tolerances which may have a negative effect on control behavior.
If, consequently, to increase the resolution in the detection of the speed of electric motor, a multipole magnet is used as a signal generator, the problem arises that with rotation magnets with more than two poles, the distribution of the poles on the magnet is not exactly symmetric, but has an error of approximately 10% per sector. This error rate holds in general for all signal generators for speed detection sensors which cannot be manufactured exact enough and operate with an optoelectric, inductive, capacitive sensor, etc. as the signal receiver.
The tolerances described and manufacturing-related errors from section to section of the signal generator or from sector to sector in a circular disk-shaped signal generator result in misinterpretations in the signal evaluation. For example, due to misinterpretations, a drop in speed is detected although the drive is operated at a constant speed, and possibly, erroneous reactions of the control arrangement of the adjustment device result, for example, an erroneous reversing of a window pane due to defective detection of a speed sensor results, which is interpreted as an entrapment situation by an entrapment protection device.